The present invention relates to ground fault circuits, and more particularly to an improved ground fault circuit that is less prone to failure.
In modern electric power generation and distribution systems, polyphase alternating current (AC) is typically generated and distributed. A number of AC sources producing equal voltages at the same frequencies, at fixed but different phase angles provide the power. In an n-phase system, n voltage source are interconnected. Each voltage source produces a sinusoidally varying voltage of fixed magnitude. The phase angle associated with each generated voltage source varies from another by 2xcfx80/n radians.
Conveniently, the n voltage sources may be interconnected to each other at a common point. Modern power distribution systems are typically three phase. In a three phase system, voltage sources and sinks that are connected at a common point are said to be connected in a xe2x80x9cwye configurationxe2x80x9d or a xe2x80x9cstar configurationxe2x80x9d. Alternatively, in a three phased circuit the voltage source or sinks may be connected in a xe2x80x9cdelta configurationxe2x80x9d.
While it is possible to interconnect multiple sources in poly-phase systems in a number of ways, the wye configuration is generally desirable in three phased systems. Specifically, for safety and other reasons, it is desirable to electrically connect the poly-phase system to ground. Wye connected sources provide a logical connection point for ground, namely the common or neutral point of the n voltage sources.
As is well understood, if one of the loads suffers a fault, caused for example by a machine fault, an excess amount of current is drawn by a single phase of the circuit. This excess current may impact on the current provided to the remaining phases. If the common point of the circuit is connected to ground much of the fault current will flow from or to the ground connection. In order to limit the amount of fault current flowing from or to ground, the neutral point is often connected to ground by way of a ground fault circuit that limits the current. Such a ground fault circuit may be formed by an impedance that limits the current through the neutral point.
Typically, such a ground fault circuit provides a single path for ground fault current to travel from the neutral point from or to ground. Moreover, typically such a ground fault circuit is unmonitored. Detection of a failure of the ground fault circuit is thus difficult. Of course, in the absence of a functioning ground fault circuit ground fault current may again significantly impact on the provision of current to functioning phases of the system or result in unlimited current to ground.
Accordingly, an improved ground fault circuit that may be monitored and that is robust is desirable.
In accordance with an aspect of the present invention, a ground fault circuit provides two current limiting paths from a neutral of a poly-phase circuit to ground. A sensing circuit monitors the paths and may indicate failure of either path. The sensing circuit, may, for example sense the net impedance of both paths and note an alarm in the event of an increased impedance, indicative of an open circuit along either path.
Advantageously, the two paths provide redundancy. In the event one path fails, the other continues to provide a path for ground fault current, and thereby limits total current to ground.
Similarly, in accordance with another aspect of the present invention, a ground fault circuit provides a plurality of current limiting paths from a neutral of a poly-phase circuit to ground. A sensing circuit monitors the plurality of paths and may indicate failure of one path. The sensing circuit, may, for example sense the net impedance of all paths and note an alarm in the event of an increased impedance, indicative of an open circuit along a path.
In accordance with yet another aspect of the present invention, a method of sensing failure of a ground fault circuit includes monitoring a net impedance of the ground fault circuit having a plurality of parallel current paths, in operation; and sensing failure of the ground fault circuit upon monitoring a change in impedance of the ground fault circuit indicative of a failure of at least one of the current paths.